refactor: declarative transform pipelines for schema generation

Restructure generate-schemas.ts with clear separation of concerns:

- TYPE_TRANSFORMS: Adapt spec types for SDK (extends clauses, extensions)
- TYPE_CLEANUP_TRANSFORMS: Prepare for export (remove index sigs, create unions)
- SCHEMA_TRANSFORMS: Post-process ts-to-zod output for Zod v4

Key improvements:
- `named()` helper creates programmatic names for parameterized transforms
  (e.g., `extendsClause(JSONRPCRequest→Request)`)
- `applyTransforms()` runs pipelines with consistent logging
- Transforms are declarative arrays, easy to reorder/add/remove
- Clear 3-phase pipeline visible in main()

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: ochafik